Windrusher:
I have no real idea how many non-PPL UK glider pilots have R/T licences. I am sure is is growing, but I think there are perhaps higher proportions of PPL and ATPL holders gliding than there used to be, too. Just an impression, based on people I meet and see correspondence from.
I try not to need to use lift in places obviously likely to interact with major civil or military traffic. If it is the "only" option, or very much better than anything else, I call them if possible, or leave it and risk landing out.
E.g. 1 - Flew from Devon to Duxford and back to Dunstable at the end of Comp. Enterprise a few years ago - turned Duxford a bit low, only visible lift was edge of Stansted Zone. I called up, explained I wanted to manouevre to gain height up to 2 miles into their zone, got the OK. Climbed enough to glide to N of Luton. Then called Luton, glider pilot ATCO said OK to do a direct glide into Dunstable via their airspace.
E.g. 2, 2 weeks ago turned Newmarket, went on track towards Great Ashfield but only apparent lift was in the edge of Honington/Mildenhall MATZ. At 1500 feet I decided I could not handle the workload of both calling and thermalling accurately, so turned back away from the MATZ, found no more lift, and landed on the gallops near Newmarket (OK to do that after 1300, choosing the right bit on a firm ground day).
Also, I sometimes use a lower bit of thermal under an approach path or similar and leave it after gaining enough height to go look somewhere less contentious before I get high enough to cause the others a problem. I have done it from above too - gone for a thermal over an ATZ, and then turned back to seek or wait for another one if it looked like I would end up dropping down into the ATZ.